1. Field of the Invention
This invention relates to a method and apparatus for wrapping a tape around a pipe.
2. Summary of the Prior Art
It is well known to provide a pipe with a tape wrapping. Usually a polymeric tape is wrapped in a spiral overlapping manner around the pipe in order to provide environmental protection. A sealing material, eg. a hot melt adhesive, may be provided between the pipe and tape, by placing it on the pipe before it is wrapped or on the interior surface of the tape or both. The relative movement of the tape and the pipe required during the wrapping can be provided by spiralling a tape supply around a fixed pipe, by rotating a tape supply around a pipe which is advanced axially only, or by pulling tape from a fixed tape supply onto a pipe which is simultaneously advanced axially and rotated.
Certain types of metal pipe, especially those having relative large diameters, eg. greater than 18 inch, have a longitudinal weld bead protruding from the surface thereof (eg. double submerged arc weld, DSAW, pipes). Such pipes present problems when tape wrapped, because the tape tends to form a tent over the weld bead, leaving two longitudinal voids, one each side of the weld bead. These voids are undesirable, especially because they provide sites for accumulation of moisture. Various methods have been proposed for overcoming this difficulty, but all suffer from serious disadvantages. For example, attempts have been made to use rollers to press the tape wrapping into the grooves on either side of the weld bead, but most tape wraps are too resilient for this to be successful. In another method the grooves are filled with a sealing material, before the tape wrap is applied, by forwarding the pipe axially only (i.e. not simultaneously rotating it), applying a viscous liquid sealing material to the area around the weld bead and doctoring the sealing material by means of a doctor blade placed generally at right angles to the axis of the pipe; the tape is then applied by spiralling a tape supply around the pipe as it is forwarded axially only. This method requires that the weld bead be accurately positioned before the process begins. Furthermore it is much more difficult to obtain good tape wrapping by spiralling the tape supply around the pipe then when the pipe is rotated, especially when the pipe and the tape are heated. The tape wrapping can be performed in a separate operation in which the pipe is simultaneously rotated and forwarded, but this is inconvenient, and furthermore the tires (or other means) used to rotate the pipe deform the sealant which has been doctored into the grooves, especially if the pipe and sealant are heated.